Overt security elements including watermarks, metallic threads and optically variable devices such as holographic foils have been used for some time to authenticate documents, bank notes and other financial transaction instruments, such as credit and debit cards, for protection against copying and counterfeiting. Such security elements are classified as Level 1 in that their presence is visible to the naked eye. Level 2 security features, such as those which have luminescent properties are also used for authentication. In this case, the security feature is normally hidden under ambient light and is only revealed to the naked eye when illuminated by a special light source such as a UV lamp. Level 3 security features may also include features which can only be detected by a machine, such as those which emit outside the visible spectrum or are based on magnetic or electrical properties of a material.
Security features may be classified as “human unassisted” or Level 1, in which the security feature is visible to the naked eye and can be authenticated by a human without machine assistance, “human assisted” or Level 2 which is defined as one in which the authentication process is performed by a person with the assistance of a tool or device, and “machine readable” in which the security feature is both detected and its authentication processed by a machine.
One of the most common Level 2 human assisted features found on bank notes is the ultra-violet (UV) fluorescent feature. This feature is typically applied as an ink, which may be visible or invisible, by offset printing and usually forms an image made up of one to three colours (red, green, blue). The image is detected by exposure of the bank note to a UV light source (typically UVA at 365 nm). This feature has provided a reasonably good level of security against most primitive and hobbyist type counterfeiting in the past.
The current fluorescent feature found on many currencies typically consists of a fluorescent pigment added to an offset ink either in a coloured or colourless ink base. A fluorescent image is revealed when the bank note is inspected with a UVA lamp (365 nm). Although this feature has proven to be an effective level 2 security feature (authentication requiring a mechanical or special equipment aid), it is under increasing threat in recent times. There is therefore a need for an improved UV fluorescent security feature that can operate with the current installed base of UV lamps found in the retail setting.
Accordingly, there is a need for an alternative security feature which is more robust against copying and counterfeiting.